Cycle-by-cycle current limit for a power tool having a brushless motor

ABSTRACT

A power tool is provided including a brushless electric motor, a switching arrangement having motor switches and interposed between the electric motor and a power supply, and a controller configured to control a switching operation of the motor switches for driving the electric motor and enforce a current limit on the current delivered to the electric motor. The controller receives a measure of current passing from the power supply to the switching arrangement and takes corrective action to reduce current delivered to the electric motor if the measured current exceeds the current limit. The controller is further configured to initiate a protective response if a number of time intervals in which the measured current exceeds the current limit exceeds a predetermined threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. application Ser.No. 14/715,079 filed May 18, 2015, which is a continuation-in-part ofU.S. patent application Ser. No. 14/057,003 filed on Oct. 18, 2013, andalso claims the benefit of U.S. Provisional Application No. 61/994,953,filed on May 18, 2014 and U.S. Provisional Application No. 62/000,307,filed on May 19, 2014. The entire disclosure of each of the aboveapplications are incorporated herein by reference.

FIELD

The present disclosure relates to a powered apparatus and more generallyto a power tool having a brushless electric motor.

BACKGROUND

Handheld angle grinders are commonly used for cutting, grinding,sanding, and polishing workpieces. Due to the large diameter of thegrinding disk and/or the large bias applied by the tool operator duringcertain tasks, the current demand by the tool from an AC power outletcan exceed the rating of the circuit breaker associated with the poweroutlet, thereby causing the breaker to trip. Consequently, there is aneed to increase the power output of such grinders and other handheldpower tools within the limits of the AC power source and withoutresorting to complicated and expensive power conversion circuits.Various techniques are set forth below for increasing power output byhandheld power tools, especially ones employing a brushless electricmotor.

This section provides background information related to the presentdisclosure which is not necessarily prior art.

SUMMARY

This section provides a general summary of the disclosure, and is not acomprehensive disclosure of its full scope or all of its features.

A handheld AC power tool is provided. The power tool is comprisedgenerally of: a brushless electric motor; a power cord connectable to anAC power socket; a converter circuit configured to receive input powerfrom the power cord and operable to output a DC bus voltage, a switchingarrangement having a plurality of motor switches and interposed betweenthe electric motor and the converter circuit; a motor drive circuitinterfaced with the motor switches to control switching operation of themotor switches; and a power switch electrically connected between theconverter circuit and the motor drive circuit and operable by a user toselectively energize the motor drive circuit and thereby power on thetool. The converter circuit includes a rectifier and a capacitorelectrically coupled across the rectifier, such that the capacitor hascapacitance sized to produce a DC bus voltage whose magnitude from an ACpower source is substantially same as magnitude of voltage from a DCpower source, where voltage rating of the AC power source is same asvoltage rating of the DC power source.

In one embodiment, the capacitor has a nominal capacitance in the rangeof 200-400 micro Farads for tool having a current rating ofapproximately 10 Amps.

In another embodiment, the capacitor has a nominal capacitance on theorder of 350 micro Farads for tool having a current rating ofapproximately 15 Amps.

In yet another embodiment, the capacitor has a nominal capacitance onthe order of 500 micro Farads for tool having a current rating ofapproximately 20 Amps.

The power tool may further include a controller configured to receive ameasure of instantaneous current passing from the rectifier to theswitching arrangement and operate over periodic time intervals toenforce a limit on current delivered to the electric motor. Thecontroller enforces the current limit by comparing instantaneous currentmeasures to the current limit and, in response to an instantaneouscurrent measure exceeding the current limit, turning off the motorswitches for remainder of a present time interval and thereby interruptcurrent flowing to the electric motor. The controller further operatesto turn on select motor switches at end of the present time interval andthereby resumes current flow to the electric motor.

In another aspect of this disclosure, a handheld grinder is provided.The grinder is comprised of: an elongated housing having a grip portionthat is shaped to be grasped by a user; a brushless electric motordrivably connected to an output shaft to impart rotary motion thereto; agrinding disk connected to one end of the output shaft; a rectifierconfigured to receive power from an alternating current (AC) powersource and operable to convert an alternating current to a directcurrent; a switching arrangement having a plurality of motor switchesand interposed between the electric motor and the rectifier; a capacitorelectrically coupled across the rectifier and interposed between therectifier and the switching arrangement; a motor drive circuitinterfaced with the motor switches and operates at a given frequency tocontrol switching operation of the motor switches; and a controllerconfigured to receive a measure of instantaneous current passing fromthe rectifier to the switching arrangement and operates over periodictime intervals to enforce a current limit on the current delivered tothe electric motor.

In some embodiments, the controller enforces the current limit bycomparing instantaneous current measures to the current limit and, inresponse to an instantaneous current measure exceeding the currentlimit, turning off the motor switches for remainder of present timeinterval and thereby interrupt current flowing to the electric motor,where duration of each time interval is fixed as a function of the givenfrequency at which the electric motor is controlled by the controller.Additionally, the controller turns on select motor switches at the endof the present time interval and thereby resumes current flow to theelectric motor. The duration of each time interval is approximately tentimes an inverse of the given frequency at which the electric motor iscontrolled by the controller. More specifically, the duration of eachtime interval is on the order to 100 microseconds.

In one embodiment, the electric motor is controlled by pulse widthmodulated (PWM) signals received from the motor drive circuit andduration of the each time interval equals period of the PWM signals.

In some embodiments, the brushless electric motor is further defined asa three-phase DC motor and the switching arrangement is comprised of sixmotor switches, each phase of the DC motor is coupled to a high-sideswitch and a low-side switch. In this case, the controller controlsmotors motor operation using the three high-side switches and enforcesthe current limit using the three low-side switches.

The capacitor has a capacitance in range of 0-200 micro Farads andpreferably in the range of 5-20 micro Farads. In an example embodiment,the capacitor is implemented by two capacitors arranged in parallel andcollectively having a capacitance on the order of 9.4 micro Farads.

In another aspect of this disclosure, a method is provided for detectingstall condition of an output shaft driven by an electric motor of apower tool. The method includes: measuring current delivered to theelectric motor of the power tool at least once during consecutive timeintervals; determining whether a measured current exceeds a currentlimit; incrementing an event counter by one in response to adetermination that the measure current exceeds the current limit;resetting the event counter to zero in response to a determination thatthe measured current does not exceed the current limit; determiningwhether value of the event counter exceeds an event threshold; andinitiating a protective operation in response to a determination thatthe event counter exceeds an event threshold.

In response to a determination that the measure current exceeds thecurrent limit, current flow to the electric motor is interrupted for theremainder of a given time interval and resuming current flow to theelectric motor at an end of the given time interval.

The method can also include: detecting beginning of a new time interval;incrementing an interval counter by one in response to detecting thebeginning of a new time interval; determining whether value of theinterval counter exceeds an interval threshold; and determining whethervalue of the event counter exceeds an event threshold in response to adetermination that the value of the interval counter exceeds theinterval threshold.

In some embodiments, the method can include: monitoring rotational speedof the electric motor; determining whether the rotational speed is lessthan a speed threshold; and initiating the protective operation inresponse to a determination that the event counter exceeds an eventthreshold and a determination that the rotational speed is less than thespeed threshold.

Further areas of applicability will become apparent from the descriptionprovided herein. The description and specific examples in this summaryare intended for purposes of illustration only and are not intended tolimit the scope of the present disclosure.

DRAWINGS

The drawings described herein are for illustrative purposes only ofselected embodiments and not all possible implementations, and are notintended to limit the scope of the present disclosure.

FIG. 1 is a perspective view of a handheld grinder;

FIG. 2A is a block diagram of an example embodiment of a motor controlscheme for use in the grinder;

FIG. 2B is a block diagram of an alternative embodiment of a motorcontrol scheme which may be used in the grinder;

FIG. 2C is a block diagram of another embodiment of a motor controlscheme which may be used in a grinder;

FIG. 3 is a partial schematic depicting an example switching arrangementfor use in the grinder;

FIG. 4A is a diagram illustrating the DC bus voltage obtained from afully-rectified AC input voltage with no load;

FIG. 4B is a diagram illustrating the DC bus voltage obtained from afully-rectified AC input voltage at a fixed load;

FIG. 4C is a diagram illustrating an idealized AC current drawn from anAC input voltage with a relatively large capacitance;

FIG. 4D is a diagram illustrating an idealized AC current drawn from anAC input voltage with a relatively small capacitance;

FIG. 5 is a graph illustrating the relationship between power output,average DC bus voltage and bus capacitance in an example power tool;

FIG. 6 is a graph illustrating implementation of a 20 amp cycle-by-cyclecurrent limit;

FIG. 7 is a block diagram depicting a portion of the motor controlscheme modified to support cycle-by-cycle current limits;

FIG. 8 is a flowchart illustrating an example method for implementingcycle-by-cycle current limits;

FIG. 9 is a graph depicting how the cycle-by-cycle current limit can beadjusted during tool start-up conditions;

FIG. 10 is a flowchart illustrating an example method for detecting astall condition using cycle-by-cycle current limit;

FIG. 11 is a flowchart illustrating an example method for detecting astall condition using cycle-by-cycle current limit; and

FIG. 12 is a graph depicting how the cycle-by-cycle current limit can beadjusted in accordance with motor temperature.

Corresponding reference numerals indicate corresponding parts throughoutthe several views of the drawings.

DETAILED DESCRIPTION

Example embodiments will now be described more fully with reference tothe accompanying drawings.

FIG. 1 depicts an example of a handheld grinder 10. In this exampleembodiment, the grinder 10 is comprised of a housing 12 having anelongated shape. A user can grasp the grinder 10 by placing the palm ofthe user's hand over and around the housing 12. An output member 18 ispositioned at one end 12-1 of the housing 12 and comprises a right anglegear set 20 that drives a rotating disk 22. In this example embodiment,the rotating disk 22 comprises a grinder disk. The rotating disk 22 maybe removed and replaced with a new rotating disk. For example, a user ofthe grinder 10 may replace the existing rotating disk 22 with a newrotating disk after the existing rotating disk 22 wears out. Anadjustable guard 24 may cover at least a portion of the rotating disk 22to obstruct sparks and debris generated during operation of the powertool 10.

While the present description is provided with reference to a grinder,it is readily understood that the broader aspects of the presentdisclosure are applicable to other types of power tools, including butnot limited to sander, drill, impact driver, tapper, fastener driver,and saw. For example, the power tool 10 may include a chuck that isconfigured to receive a drill bit or a screw driver bit, therebyallowing the power tool 10 to be used as a power drill or a power screwdriver. In another example embodiment, the output member 18 may beremoved and replaced with another output member that may be moresuitable for a drill, a screw driver, or any other power tool, thuscreating a multipurpose power tool by virtue of a plurality of outputmembers 18.

The housing 12 has a first portion 14 and a second portion 16. The firstportion 14 and second portion 16 may be secured together with screws 26,illustratively six, and enclose an electric motor 28 and electroniccircuit components, as further described below, that drive the outputmember 18. The first portion 14 further includes a power on/off switch32 and a spindle lock switch 34. Putting the power on/off switch 32 inon and off positions turns on and off the electric motor 28,respectively. Pressing and holding the spindle lock switch 34 enablesthe user to change the rotating disk 22. A plurality of narrow slotopenings 36 of the first 14 and second 16 portions allow for venting ofthe electric motor 28 and the electronic circuit components. The one end12-1 of the housing 12 also includes a threaded opening 38 forselectively attaching a side-handle (not shown) to enable two-handedoperation.

A power cord 30 is connectable to an AC power socket and is positionedat an opposite end 12-2 of the housing 12. The power cord 30 providespower to the electric motor 28 and the electronic circuit components ofthe power tool 10. Additionally, the power tool 10 is configured toreceive a detachable battery pack 42. Specifically, the housing 12includes a battery mounting portion to which one or more battery packs42 releasably couple thereto. The battery pack 42 provides DC power tothe electric motor 28 and the other electronic components of the powertool. Different attachment mechanisms for battery packs are readilyknown in the art and may be employed in this application.

In some embodiments, it is envisioned that the tool may be configured towork with battery packs having different nominal voltage ratings. Forexample, the tool may be configured to work with a single state lowvoltage pack (e.g., 20V) or a convertible low/medium voltage pack (e.g.,20V/40V or 20V/60V). In other examples, the tool may be configured towork with two convertible medium voltage packs to yield a high outputvoltage (e.g., 120V or 230V). It is understood that number andarrangement of battery cells in a pack as well as the cell chemistry mayvary. Further information regarding power tool systems and convertiblebattery packs may be found in U.S. Provisional Patent Application Nos.61/994,953 filed on May 18, 2014 and 62/000,112 filed on May 19, 2014which are incorporated herein in their entirety.

FIG. 2A depicts a schematic that illustrates an embodiment of a motorcontrol system 80 that may be employed by the power tool 10. The motorcontrol system 80 is comprised generally of the controller 78, aswitching arrangement 82 and a driver circuit 84. The motor controlsystem 80 operates to drive an electric motor 28 which may be furtherdefined as a brushless electric motor. The brushless motor may be athree-phase permanent magnet synchronous motor including a rotor havingpermanent magnets and a wound stator that is commutated electronicallyas described below. The stator windings are designated herein as U, V,and W windings corresponding to the three phases of the motor 28. Itmust be understood, however, that other types of brushless motors, suchas switched reluctance motors and induction motors, are within the scopeof this disclosure. It must also be understood that the brushless motor28 may include fewer than or more than three phases. While many of theconcepts presented herein are particularly applicable to brushlessmotors, some of the concepts can be applied to other types of motors aswell.

The motor control system 80 may further include position sensors 86, 88,90 that are configured to detect rotational motion of the electric motor28 and generate a signal indicative of the rotational motion. The signalmay have a periodic waveform whose magnitude may vary in accordance withthe rotational position of the electric motor 28. The controller 78 isconfigured to receive signals output by the position sensors 86, 88, 90.In other embodiments the position sensors 86, 88, 90 may be interfacedwith the driver circuit 84.

In the example embodiment, the power tool 10 is configured to work witheither an AC power supply 92 or a DC power supply 91. The AC powersupply 92 delivers an alternating current to the rectifier 44, forexample via the power cord 30. The rectifier 44 converts the alternatingcurrent into a direct current.

Likewise, a DC input signal from the DC power supply 92 passes throughthe rectifier 44. When powered from the DC power supply 92, therectifier 44 simply ensures that the DC supply is connected to theswitching arrangement 82 and driver circuit 84 with the correctpolarity. In one embodiment, the DC power supply is one or morerechargeable battery packs that detachably couple to the housing 12 ofthe power tool 10. In other embodiments, the power tool 10 may beconfigured to receive a DC input via the power cord 30 from other typesof sources, such as a generator, a portable welder or a DC outlet incertain types of electrical installations.

The AC power supply 92 and the DC power supply 91 must not be providingpower to the tool at the same time to prevent damage to one or both ofthe two power supplies. Accordingly, an electrical and/or mechanicalinterlock is used to ensure that only one of the power supplies isproviding power to the tool. Various forms of interlocks are readilyknown in the art and may be employed in this context. It is envisionedthat the interlock is configured to generate a signal indicative ofwhether a battery pack is coupled to the tool. In the case the tool canbe powered by battery packs having different nominal voltages, thesignal generated by the interlock may further specify the type and/ornominal voltage of the battery packs coupled to the tool.

One example embodiment of an interlock is the double-pole-double-throw(DPDT) switch 93 which connects the AC power supply 92 to the rectifier44 when it is energized. The switch is in the form of a relay, the coilof said relay energized by the closure of a second small switch thatcloses only when the AC power supply line is connected to the powertool. Thus, connection of the AC power supply line to the power toolcloses the small switch which in turn energizes the relay which drivesthe DPDT switch 93 in such a way as to connect AC power supply 92 torectifier 44. Conversely, without the AC power supply line the smallswitch is not closed, the relay is not energized, and the DPDT switch 93remains in its unenergized position connecting the DC power supply 91 tothe rectifier 44.

Another example embodiment of an interlock to cause the DPDT switch 93to connect the AC power supply 92 to the rectifier 44 is to slide amechanical shutter, said shutter covering either the opening whichallows connection of the AC power supply line, or covering the otheropening which allows connection of the DC power supply 91. In someembodiments, it is envisioned that no interlock is required because theAC power supply and DC power supply have respective protection circuitrysuch that in the event of simultaneous connection, neither power supplyis damaged.

One or more DC bus capacitors 50 are electrically connected in parallelwith the rectifier 44. In one aspect of this disclosure, the DC buscapacitors 50 are sized to ensure that the tool delivers comparableoutput power from either the AC power supply or the DC power supply 91without exceeding breaker limits as will be further described below.

In the example embodiment, the switching arrangement 82 is electricallyconnected with the DC bus capacitors 50 and may receive a pure DC signalor substantially pure DC signal from the DC bus capacitors 50. Theswitching arrangement 82 includes a plurality of motor switches that,when switched on, deliver the DC current to the electric motor 28.Example motor switches include field effect transistors (FETs),insulated-gate bipolar transistors (IGBTs), etc. In the exampleembodiment, the switching arrangement 82 may be further defined as athree-phase inverter bridge as shown in FIG. 3. As shown, thethree-phase bridge circuit includes three high-side and three low-sideFETs. The gates of the high-side FETs driven via signals UH, VH, and WH,and the gates of the low-side FETs are driven via motor control signalsUL, VL, and WL. In an embodiment, the sources of the high-side FETs arecoupled to the drains of the low-side FETs to output power signals PU,PV, and PW for driving the electric motor 28. Other switchingarrangements are also contemplated by this disclosure.

The driver circuit 84 is interfaced with the motor switches of switchingarrangement 82. The driver circuit 84 controls the state of the motorswitches, for example using pulse width modulated (PWM) control signals.In this embodiment, the driver circuit 84 is shown as being separatefrom the switching arrangement 82. In other embodiments, the drivercircuit 84 and the switching arrangement 82 may be a single integratedcircuit which may be commercially available from various manufactures.For example, the switching arrangement 82 and the driver circuit 84 maybe a part of an integrated power module.

The controller 78 manages the overall operation of the tool. Forexample, the controller controls switching operation of the motorswitches in the switching arrangement. In one embodiment, the controlleris implemented by a microcontroller. Controller may also refer to anelectronic circuit, an application specific integrated circuit (ASIC), aprocessor (shared, dedicated, or group) and/or memory (shared dedicated,or group) that execute one or more software or firmware programs, acombinational logic circuit, and/or other suitable components thatprovide the described functionality.

In this embodiment, the controller 78 receives power from a driver powersupply 94. The driver power supply 94 is electrically connected inseries with the rectifier 44 and operates to power the driver circuit 84via the power on/off switch 32. In an example embodiment, driver powersupply 94 includes a buck converter and/or a linear regulator to reducethe power voltage, for example, to 15V for powering the driver circuit84 and to 3.2V for powering the controller 78. In an alternateembodiment, the controller 78 may receive power directly from therectifier 44.

The power on/off switch 32 is positioned between the driver power supply94 and the driver circuit 84. In an example embodiment, the switchcontact of the power on/off switch 32 is positioned between the driverpower supply 94 and the driver circuit 84. In other embodiments, thepower switch 32 may be implemented by a variable speed actuator.

When the power on/off switch 32 is switched to the on position, thedriver circuit 84 receives power from the driver power supply 94. Whenthe driver circuit 84 receives power, the driver circuit 84 is able tocontrol the state of the motor switches and the electric motor 28 is on.

Conversely, when the power on/off switch 32 is switched to the offposition, the driver circuit 84 does not receive power from the driverpower supply 94. When the driver circuit 84 does not receive power, thedriver circuit 84 is not able to control the state of the motor switchesand the electric motor 28 is off.

As illustrated, the power on/off switch 32 is electrically connectedbetween the rectifier 44 and the driver circuit 84. The power on/offswitch 32 is positioned such that the power, conveyed from either powersupply through the switching arrangement 82, does not pass through thepower on/off switch 32. Furthermore, the current being drawn by theelectric motor 28 does not pass through the power on/off switch 32. Thecurrent passing through the power on/off switch 32 is the current beingdrawn by the driver circuit 84 and the current being drawn by the drivercircuit 84 is lower than the current being drawn by the electric motor28.

The power on/off switch 32 has a current rating that is approximatelyequal to the lower current being drawn by the driver circuit 84 and notthe higher current being drawn by the electric motor 28. Similarly, thepower on/off switch 32 has a voltage rating that is approximately equalto the lower voltage at which the driver circuit 84 operates and not thehigher voltage at which the electric motor 28 operates. The power on/offswitch 32 is a low current and low voltage switch. Advantageously, thepower on/off switch 32 has smaller physical dimensions and generatesless heat than a switch that would be required to withstand the highercurrent and higher voltage at which the electric motor 28 operates.

FIG. 2B depicts an alternative embodiment of a motor control system 80that may be employed by the power tool 10. In this embodiment, theoutput from the DC power supply 91 bypasses the rectifier 44 and isinjected directly across the DC bus capacitors 50. By bypassing thevoltage drop across the diodes of the rectifier 44, this approachincreases the efficiency for a DC power supply. DC polarity is criticalin this embodiment as the opposite polarity would damage the switchingarrangement. It is also noted that the interlock is positioned beforethe rectifier 44 but could also be placed after the rectifier 44 aswell. Except with respect to the differences discussed herein, thisembodiment is substantially the same as the one described above inrelation to FIG. 2A.

In some embodiments, the handheld grinder 10 may be configured toreceive power only from an AC power source. That is, the grinderincludes a power cord for connecting to an AC outlet but does notinclude a mounting portion for a battery pack. FIG. 2C depicts a motorcontrol system 80 for such a power tool. In this variant, the rectifier44 is configured to receive an alternating current directly from an ACpower supply without the need for an interlock. Except with respect tothe differences discussed herein, this motor control system issubstantially the same as the one described above in relation to FIGS.2A and 2B.

Brushless DC power tools have been adapted to AC applications by simplyrectifying the AC power into DC power. This can be accomplished, forexample by means of a full-wave bridge rectifier followed by a suitablecapacitor, and the use of higher voltage electronic components. Higherpower output in brushless AC power tools also requires increases in thesteady-state current carrying capacity of the electronics and theability to remove additional waste heat. With these changes, brushlessDC motors can be adapted to AC power sources.

Because brushless DC power tools are most often operated from arechargeable battery, their power source is a relatively constant DCvoltage, namely the battery voltage. With the advent to brushless ACpower tools, the rectifier and capacitor were sized accordingly toproduce a relatively constant DC voltage from which the brushless DCmotor could operate as shown in FIG. 4A. The full-wave rectified ACwaveform as measured across the capacitor 50 is referred to as the “DCbus voltage”. It is noted that in this diagram, it is assumed that thetool is operating under a maximum heavy load that the tool is rated tohandle.

Constant, or relatively constant, voltage is typically achieved withlarge values of capacitance following the full-wave bridge rectifier.For discussion purposes, the comparatively small voltage drops acrossthe individual diodes in the full-wave bridge rectifier are ignored. Thenumerical value of the capacitor will be determined by the currentdemands of the motor under load, and the allowable voltage ripple on thecapacitor. Voltage ripple on the capacitor means there is ripple currentin the capacitor, and because all real capacitors have some (but notzero) equivalent series resistance (ESR), this ripple current will causeheating inside the capacitor. Small amounts of heat are acceptable butlarge quantities of heat generated inside the capacitor are notacceptable and may cause damage.

For a given load, the larger the numerical value of capacitance, thelower the ripple voltage as seen in FIG. 4B. The ripple current is equalto the value of capacitance times the time derivative of the ripplevoltage across the capacitor, so long as the equivalent seriesresistance is very small compared to the reactance. A larger capacitorcan store more charge and deliver more current, with less voltage rippleand lower equivalent series resistance, than a smaller capacitor. Thus,large values of capacitance, resulting in DC voltages with very lowripple, were the first choice when making a brushless AC power tool witha brushless DC motor. They most resemble the batteries for whichbrushless motors were designed.

There are two important consequences of a large capacitor—a smallerpower factor for the power tool and less work that can be performed withan AC circuit breaker of fixed root-mean-square (RMS) current limit.Because the capacitor is large in value and the voltage ripple is low,the DC voltage across it is considered “stiff”. As the power tool drawscurrent from the DC Bus, it will come from the charge stored on thecapacitor, if the instantaneous magnitude of the AC voltage is notsufficient in the moment to supply it. The capacitor's charge will thenbe replenished from the AC power source through the full-wave bridgerectifier when the magnitude of the AC voltage later rises slightlyabove the voltage across the capacitor, and the DC Bus voltage will thenfollow the AC voltage up to its maximum as shown in FIG. 4B. Saiddifferently, when the instantaneous magnitude of the AC voltage is lowerthan the DC Bus voltage, the capacitor will supply the current requiredby the motor. The DC Bus voltage will decrease accordingly, so long asthe instantaneous magnitude of the AC voltage is lower. At the pointwhere the DC Bus voltage equals the instantaneous magnitude of the ACvoltage, the DC Bus voltage will cease decreasing and begin to followthe AC voltage upward. At this point, the current demands of the motorare supplied directly by the AC power source.

With large values of capacitance, the motor current is usually suppliedby the capacitor. In the short period where the instantaneous magnitudeof the AC voltage is higher than the DC Bus voltage, current will rushfrom the AC source to replenish the charge on the capacitor as well asto power the motor. Thus, large spikes of current are drawn from the ACsource near the maxima and minima of the AC voltage as seen in FIG. 4C.

The spiky nature of the AC current is undesirable for two reasons.First, it means that the power factor of the power tool is low and theharmonic content of the AC current is high. The second undesirableresult is that for a high value of output power delivered by the powertool the RMS value of the AC input current will be high. The practicalresult is that an unnecessarily large AC circuit breaker is requiredwith such spiky AC currents for a given amount of work.

Many AC circuits are protected, for example by 15 amp RMS circuitbreakers. If the AC current is extremely spiky, as it is with a largevalue of capacitor, then relatively little work can be performed. If theAC current were lower in value and more spread out in time then muchmore work could be performed from the same 15 Amp circuit.

One technique for spreading out the AC current drawn from the AC sourceis to use a relatively small value of capacitance for the DC buscapacitors 50. For example, a high power output tool (e.g., 1.7kilowatts) with a brushless motor will have its nominal capacitance forthe DC bus capacitors 50 in the range of 15-20 μF. For low power outputtools, the nominal capacitance for the DC bus capacitors may be setlower. For example, a nominal capacitance of 10 μF may be suitable for atool having a power output of 0.85 kilowatts. Thus, the capacitancevalue is dependent upon the output power requirements of the tool. It isenvisioned that the nominal capacitance is in the range of 0-200 μF andpreferably in the range of 5-20 μF.

In an example embodiment, the DC bus capacitors 50 are implemented usingfilm capacitors. Film capacitors can be made from polymer plasticsmetalized on both sides and may be rolled with additional suitableinsulators. The DC bus capacitors 50 may be implemented by a singlecapacitor or a series of capacitors (e.g., two 4.7 μF capacitors inparallel to achieve a nominal capacitance of 9.4 μF or four 4.7 μFcapacitors in parallel to achieve a nominal capacitance of 18.8 μF). Incomparison to traditional electrolytic capacitors, film capacitors havesmaller physical dimensions and less equivalent series resistance perunit capacitance. By reducing the nominal capacitance of the DC buscapacitors 50, the time which the AC voltage replenishes charge on thecapacitor is greatly increased. The result is that the power factorincreases, the harmonic content of the AC current goes down, and theamount of work that can be performed through a 15 Amp AC circuit breakerincreases as shown in FIG. 4D.

It is envisioned that the power cord of the power tool is connectable toa DC power source, e.g., a DC generator such as a welder having a DCoutput power line, having a DC output voltage of 120V. With a smallcapacitor 50 having a capacitance value of approximately 0-50 μF, powertool 10 may provide a higher max power out from a DC power source havinga nominal voltage of 120 VDC, than it would from a 120V AC mains powersource. Specifically, using a small capacitor of 0-50 microF, the DC busvoltage resulting from a 120V AC mains power source has a nominal valueof approximately 108V. An exemplary power tool may provide a maximumcold power output of approximately 1600 W from the 108V DC bus voltage.By comparison, the same power tool provides a maximum cold power outputof more than 2200 W from the DC bus when power is being supplied by the120V DC power source. This improvement represents a ratio of2200/1600=1.37 (which corresponds to the voltage ratiô3, i.e.,(120/108)³).

In another aspect of this disclosure, comparable power outputs from theAC and DC power sources can be achieved by adjusting the capacitancevalue of the capacitor 50. FIG. 5 depicts power output and average DCbus voltage in relation to capacitance values for capacitor 50. The xaxis in this diagram depicts varying capacitance values from 0 to 1000uF; whereas, the Y axes respectively represent the maximum powerwatts-out (W) of the power tool ranging from 0-2500 W as well as theaverage DC bus voltage (V) ranging from 100-180V. The three RMS currentvalues represent the rated RMS current of the AC power supply. Forexample, in the US, the wall socket may be protected by a 15 A RMScurrent circuit breaker. In this example, it is assumed that the powertool is operating under heavy load close to its maximum current rating.

As shown in this diagram, for a power tool configured to be powered by a10 A RMS current power supply (i.e., the tool having a current rating ofapproximately 10 A RMS current, or a power supply having a currentrating of 10 A RMS current), the average DC bus voltage under heavy loadis in the range of approximately 108-118V for the capacitor range of0-200 uF; approximately 118-133V for capacitor range of 200 to 400 uF;approximately 133-144V for capacitor range of 400-600 uF, etc.

Similarly, for a power tool configured to be powered by a 15 A RMScurrent power supply (i.e., the tool having a current rating ofapproximately 15 A RMS current, or a power supply having a currentrating of 15 A RMS current), the average DC bus voltage under heavy loadis in the range of approximately 108-112V for the capacitor range of0-200 uF; approximately 112-123V for capacitor range of 200 to 400 uF;approximately 123-133V for capacitor range of 400-600 uF, etc.

Similarly, for a power tool configured to be powered by a 20 A RMScurrent power supply (i.e., the tool having a current rating ofapproximately 20 A RMS current, or a power supply having a currentrating of 20 A RMS current), the average DC bus voltage under heavy loadis in the range of approximately 108-110V for the capacitor range of0-200 uF; approximately 110-117V for capacitor range of 200 to 400 uF;approximately 117-124V for capacitor range of 400-600 uF, etc.

In one embodiment, in order to provide an average DC bus voltage fromthe AC mains power source (e.g., a 108V nominal RMS voltage) that iscomparable to the nominal voltage received from the DC power source (120VDC), the capacitor value may be sized based on the current rating ofthe power tool and the target DC bus voltage. For example, a capacitorvalue of approximately 230 uF may be used for a tool powered by a 10 ARMS current power supply (i.e., the tool having a current rating ofapproximately 10 A RMS current, or configured to be powered by a powersupply having a current rating of 10 A RMS current) to provide anaverage DC bus voltage of approximately 120V from the AC mains. Thisallows for the power tool to provide substantially similar output levelsfor 120V AC power source as it would from a 120V DC power source.

Likewise, a capacitor value of approximately 350 uF may be used for atool powered by a 15 A RMS current power supply (i.e., the tool having acurrent rating of approximately 15 A RMS current, or configured to bepowered by a power supply having a current rating of 15 A RMS current)to provide an average DC bus voltage of approximately 120V from the ACmains. More generally, capacitor may have a value in the range of290-410 uF for a tool powered by a 15 A RMS current power supply toprovide an average voltage substantially close to 120V on the DC busfrom the AC mains. This allows for the power tool to providesubstantially similar output levels for 120V AC power source as it wouldfrom a 120V DC power source.

Finally, a capacitor value of approximately 500 uF may be used for atool powered by a 20 A RMS current power supply (i.e., the tool having acurrent rating of approximately 20 A RMS current, or configured to bepowered by a power supply having a current rating of 20 A RMS current)to provide an average DC bus voltage of approximately 120V from the ACmains. More generally, the capacitor may have a value in the range of430-570 uF for a tool powered by a 20 A RMS current power supply toprovide an average voltage substantially close to 120V on the DC busfrom the AC mains. This allows for the power tool to providesubstantially similar output levels for 120V AC power source as it wouldfrom a 120V DC power source.

In another aspect of this disclosure, a cycle-by-cycle current limit isalso implemented in the power tool 10. When the instantaneous buscurrent in a given cycle exceeds a prescribed current limit, the selectdrive signals to the switches in the switching arrangement 82 are turnedoff from the remainder of the cycle. At the beginning of the next cycle,these drive signals are restored. For each cycle, the instantaneouscurrent continues to be evaluated in a similar manner. This principle isillustrated in FIG. 6, where the thinner line indicates theinstantaneous current without a limit and the thicker line indicates theinstantaneous current with an enforced 20 Amp limit. Cycle-by-cyclecurrent limit enables the power tool to achieve similar performanceacross different types of power sources and under varying operatingconditions as will be further described below.

Cycle-by-cycle current limiting can be implemented by modifying themotor control system 80 as shown in FIG. 7. Specifically, a currentsensor 94 is configured to sense the current through the DC bus andprovide a signal indicative of the sensed current to the controller 78.In an exemplary embodiment, the current sensor 94 is implemented using ashunt resistor disposed in series between the capacitor 50 and theswitching arrangement 82. Although not limited thereto, the shuntresistor is preferably positioned on the low voltage side of the DC bus.In another embodiment, a shunt may be placed in each lower leg of theswitching arrangement 82. In this case, current for each phase is sensedand reported to the controller 78. It is also envisioned that the shuntsmay be positioned on the input side of the rectifier as well. In anycase, the controller 78 is able to detect the instantaneous currentbeing delivered to the motor.

The controller 78 is configured to receive a measure of instantaneouscurrent passing from the rectifier to the switching arrangement operatesover periodic time intervals (i.e., cycle-by-cycle) to enforce a currentlimit. With reference to FIG. 8, the controller enforces the currentlimit by measuring current periodically (e.g., every 5 microseconds) at801 and comparing instantaneous current measures to the current limit at802. If the instantaneous current measure exceeds the current limit, thecontroller 78 cooperatively operates with the driver circuit 84 to turnoff the motor switches for remainder of present time interval at 803 andthereby interrupt current flowing to the electric motor. If theinstantaneous current measure is less than or equal to the currentlimit, the controller 78 continues to compare the instantaneous currentmeasures to the current limit periodically for the remainder of thepresent time interval as indicated at 804. Such comparisons preferablyoccur numerous times during each time interval (i.e. cycle). When theend of the present time interval is reached, the controller 78cooperatively operates with the driver circuit 84 to turn on the motorswitches at 805 and thereby resume current flow to the motor for thenext cycle. In one embodiment, the duration of each time interval isfixed as a function of the given frequency at which the electric motoris controlled by the controller. For example, the duration of each timeinterval is set at approximately ten times an inverse of the frequencyat which the electric motor is controlled by the controller. In the casethe motor is controlled at a frequency of 10 kilo-Hertz, the timeinterval is set at 100 microseconds. In other embodiments, the durationof each time interval may have a fixed value and no correlation with thefrequency at which the electric motor is controlled by the controller.

In the example embodiment, the electric motor is controlled by pulsewidth modulated (PWM) signals received from the motor drive circuit andduration of the each time interval equals period of the PWM signals. Ina fixed speed tool under a no load condition, the duty cycle of the PWMcontrol signals is set, for example at 60%. Under load, the controller78 operates to maintain a fixed speed by increasing the duty cycle. Ifthe current through the DC voltage bus increase above the current limit,the controller 78 interrupts current flow as described above which ineffect reduces the duty cycle of the PWM signals. For a variable speedtool under a no load condition, the duty cycle of the PWM controlsignals ranges for example from 15% to 60%, in accordance with usercontrolled input, such as a speed dial or a trigger switch. Thecontroller 78 can increase or decrease the duty cycle of the PWM signalsduring a load condition or an over current limit condition in the samemanner as described above. In one embodiment, speed control and currentlimiting may be implemented independently from each other by using threeupper motor switches for speed control and the three lower switches forcurrent limiting. It is envisioned that the two functions may be swappedbetween the upper and lower switches or combined together into one setof switches.

In the examples set forth above, the time interval remained fixed. Whenthis period (time interval) remains fixed, then the electronic noisegenerated by this switching will have a well-defined fundamentalfrequency as well as harmonics thereof. For certain frequencies, thepeak value of noise may be undesirable. By modulating the period overtime, the noise is distributed more evenly across the frequencyspectrum, thereby diminishing the noise amplitude at any one frequency.In some embodiment, it is envisioned that the time interval may bemodulated (i.e., varied) over time to help distribute any noise over abroader frequency range.

In a variant, the controller 78 enforces the cycle-by-cycle currentlimit by setting or adjusting the duty cycle of the PWM drive signalsoutput from the gate driver circuit 84 to the power switch circuit 82.In an embodiment, the duty cycle of the PWM drive signals may beadjusted in this manner following the instant current cycle (i.e., atthe beginning of the next cycle). In a fixed speed tool, the controller78 will initially set the duty cycle of the drive signals to a fixedvalue (e.g., duty cycle of 75%). The duty cycle of the drive signalswill remain fixed so long as the current through the DC bus remainsbelow the cycle-by-cycle current limit. The controller 78 willindependently monitor the current through the DC bus and adjust the dutycycle of the motor drive signals if the current through the DC busexceeds the cycle-by-cycle current limit. For example, the controller 78may lower the duty cycle to 27% to enforce the 20 amp current limit. Inone embodiment, the duty cycle value may be correlated to a particularcurrent limit by way of a look-up table although other methods forderiving the duty cycle value are contemplated by this disclosure. Forvariable speed tool, the controller 78 controls the duty cycle of themotor drive signals in a conventional manner in accordance with thevariable-speed signal from the variable-speed actuator. Thecycle-by-cycle current limit is enforced independently by the controller78. That is, the controller will independently monitor the currentthrough the DC bus and adjust the duty cycle of the drive signals onlyif the current through the DC bus exceeds the cycle-by-cycle currentlimit as described above.

In some embodiments, the cycle-by-cycle current limit may be dependentupon the type and/or nominal voltage of the power supply. For a toolconfigured to receive power from either a DC or AC power supply, thecontroller 78 may receive a signal from the interlock mechanism 96,where the signal indicates whether the tool is coupled to a DC supply(e.g., 120V) from a battery pack or an AC supply (e.g. 120V) from an ACsocket. In lieu of or in addition to the DC supply, the tool may befurther configured to receive power from an AC source having differentnominal voltages (e.g., 120 v or 230V). Likewise, the tool may beconfigured to receive power from battery packs having different nominalvoltages (e.g., 60V or 120V). In these cases, the controller 78 alsoreceives a signal indicative of the amplitude of the input power signal.In other embodiments, the controller may be configured to sense directlythe nominal voltage being supplied by the power supply. Other methodsfor determining the type and/or nominal voltage of the power supply arealso contemplated by this disclosure.

Given the type and/or nominal voltage of the power supply, thecontroller 78 selects a current limit to enforce during operation of thepower tool. In one embodiment, the current limit is retrieved by thecontroller 78 from a look-up table. An example look-up table is asfollows:

Source type Nominal voltage Current limit AC 120 V 40 A AC 230 V 20 A DC120 V 35 A DC 108 V 40 A DC  60 V 70 A DC  54 V 80 AThat is, the controller 78 will enforce a 40 Amp current limit when thetool is coupled to a 120V AC power supply but will enforce a 20 Ampcurrent limit when the tool is coupled to a 230V AC power supply. As aresult, the effective output power of the tool is substantially thesame. Because the average voltage supplied from by a 120V AC source isapproximately 108 volts DC, the controller 78 will also enforce aslightly lower current limit (e.g., 35 amps) when the tool is coupled toa 120V DC power supply. This lower current limit results in output powerso that it is substantially the same as when the tool is coupled to 120VAC power supply. Similarly, the controller 78 will enforce a highercurrent limit (e.g., 70 amps) when the tool is coupled to a 60V DC powersupply. In this example, output power of the tool has been normalized tothe case of a 120V AC power supply. Tool performance could also benormalized in relation to one of the other types of power supplies.Moreover, it is understood that the current limits set forth above areapproximate values and more precise values may be used in practice toachieve similar performance from the tool across different powersupplies.

During motor rotation, the electronic switches in the switchingarrangement 82 transition back and forth between current conduction andno current conduction. In their conducting state they are not lossless,but generate some heat which, if not removed, will raise the temperatureof the switch to the point of failure. Limiting the amount of currentthrough the switch will limit the amount of heat generated within it.Often, extreme temperature is the reason for failure in electronicswitches. Maintaining a reasonable temperature allows the electronicswitch to continue to operate. Conventionally, this is accomplished bycooling the switches with airflow and a heat sink, and also by limitingthe current through the switches. Because electronic switches in anelectronically commutated three-phase motor typically conduct currentapproximately one-third of the time, it becomes possible to raise thelevel of current during the one-third of the time the switches areconducting because there is no conduction in the other two-thirds of thetime.

When the electronically commutated motor is first started, it beginsrotating from a stationary position and the appropriate upper and lowerelectronic switches in the switching arrangement are conductingcontinuously. Only after the motor achieves some significant rotationalspeed does each electronic switch experience the one-third duty cyclesufficient to average out the heat generation across the cycle.

To prevent the electronic switches from overheating during start-up,cycle-by-cycle current limiting may be used to limit current atstart-up. Rather than implementing a single current limit, the tool 10is configured to apply a lower current during start-up as shown in FIG.9. In this example, a current limit of 10 amps is enforced by thecontroller 78 when the shaft speed is less than a predefined threshold(e.g., 12,000 rpm). After a sufficient shaft speed has been achieved,the cycle-by-cycle current limit is raised to a high limit of 20 amps.Should the shaft speed decrease, the current limit may revert back tothe lower limit of 10 Amps. This feature may be implemented withhysteresis so that the current limit does not revert back to the lowervalue until the shaft speed is less than a second predefined thresholdwhich has a value lower than the first threshold (e.g. 7,000 rpm).Current limits and speed threshold values are merely illustrative andmay vary depending on switch conduction time and other factors. It isenvisioned that the shaft speed may be determined by the controller 78,for example from input from position sensors.

In another aspect of this disclosure, cycle-by-cycle current limitingcan be used to detect a pinch event or stall condition and possiblyprotect a user from losing control of the tool. A pinch event or stallcondition is understood to mean an event that very quickly deceleratesthe rotation of the tool and causes an immediate rise in current. In thecase of a grinder, a pinch event may occur when using the grinder as acut-off tool and the workpiece moves in such a way as to pinch thecut-off accessory. Causes for a pinch event vary with the type of tooland the type of accessory.

An example method for detecting a pinch event using cycle-by-cyclecurrent limit is further described in relation to FIG. 10. For eachcycle, the instantaneous bus current is measured periodically (e.g.,every 1 microsecond) during the cycle as indicated at 112. The measuredcurrent is compared at 113 to the current limit presently being enforcedby the controller. When the current limit is exceeded, select drivesignals to the switches in the switching arrangement 82 are turned offfor the remainder of the cycle as indicated at 114. Additionally, anevent counter is incremented by one at 115 and an over limit flag is settrue at 116.

Before checking the current limit, the over limit flag is checked atstep 111. Measurement of the current and its comparison to the currentlimit will continue for the entire cycle so long as the over limit flagis set false. Conversely, these steps are skipped for the reminder ofthe cycle once the over the limit flag is set to true. In this way, theevent counter is incremented (if applicable) only once per cycle. Thus,the check of the current limit will be repeated until either the currentlimit is exceeded or a new cycle begins.

At the beginning of the next cycle, the drive signals to the switchesare restored by the controller. Additionally, a cycle counter isincremented by one at 117 and the over limit flag is reset to false at118. Moreover, a determination is made as to whether a pinch event hasoccurred once a sufficient sample size has accrued. For example, adetermination about a pinch event can be made every 10 milliseconds.Assuming a cycle of 100 microseconds, the determination about a pinchevent would be made when the cycle counter reaches a count of 100. To doso, the cycle counter is compared at 119 to a predefined cycle thresholdwhose value correlates to the determination period (e.g., 100 in theexample above).

When the cycle count reaches the threshold value, the event counter isthen compared at 120 to an event threshold. In one embodiment, a pinchevent is deemed to have occurred when the event count exceeds the eventthreshold (e.g., 20). In this case, the controller initiates aprotective operation at 122 to protect to the tool operator. Forexample, the controller may cut power to the motor and/or apply a braketo the output shaft of the tool coupled thereto. Suspending drivesignals effectively makes the motor coast. In a pinch event, momentum istransferred to the tool and thus the user. With braking, this momentumis reduced to lower levels than experienced during coasting, which helpsto protect the user. Other types of protective operations are alsocontemplated by this disclosure.

In other embodiments, when the event count reaches the event threshold,the present 10 millisecond time interval is counted as an over-currentinterval, and a pinch event is deemed to have occurred, when a fixednumber (e.g., 10) of over-current intervals occur consecutively. Whenthe event count does not exceed the event threshold, the event counterand the cycle counter are reset at 121 and processing of the remainderof the cycle continues as indicated at 112. Likewise, when the cyclecount does not exceed the cycle threshold, processing of the remainderof the cycle continues. Values set forth above have been found to bereasonable for a wide variety of conditions but are not consideredlimiting. Different values can be used to accommodate specific type ofpinch conditions in different types of tools. It is to be understoodthat only the relevant steps of the methodology are discussed inrelation to FIG. 10, but that other software-implemented instructionsmay be needed to control and manage the overall operation of the system.

The method set forth above may also be used to detect stall conditionsas well as other tool conditions which may develop at a slower rate thana pinch event. For example, when a tool operates under heavy loads, thecurrent levels are high and the motor speed is low such that airflow islow and the tool is at risk of overheating. With reference to FIG. 11,the methodology is substantially similar to that described above inrelation to FIG. 10. A secondary tool parameter is used to detect astall condition. In addition to checking the event count, the rotationalspeed of the shaft is determined at 223 and compared to a speedthreshold at 224. A stall condition is deemed to have occurred when theevent count exceeds the event threshold and the rotational speed of theshaft is less than a speed threshold (e.g., 4,000 rpm). In thisscenario, the controller initiates a protective operation at 222 toprevent damage to the tool. The remaining steps and variants thereof aredescribed above in relation to FIG. 10 and omitted here for brevity.While reference is made to rotational shaft speed as the secondary toolparameter, other tool parameters may also be used in conjunction withover current events to detect stall conditions. One of those other toolparameters may be time since the last signal from any positionindicator. If that time, as expressed as a count of a timer, exceedssome limit, and the over current event counter also exceeds its limit,then motor operation is suspended as described above.

There are also other techniques for testing the instantaneous currentand the methods described above are not meant to limit the scope of thisdisclosure. In the examples above the current is measured at discreteintervals. But analog comparators can also be used to signal aninstantaneous over current limit and these signals can then be used insimilar fashion to effect the same result as described above. It isagain to be understood that only the relevant steps of the methodologyare discussed in relation to FIG. 11, but that othersoftware-implemented instructions may be needed to control and managethe overall operation of the system.

Adjustments to the cycle-by-cycle current limit may also be used toprevent overheating conditions from occurring in the tool. Variablespeed grinders present a special challenge—preventing burn up underheavy loads at low speeds. Variable speed of a variable speed grindermay be set, for example by a rotary thumbwheel. Ranging from high speedselection to low speed selections, the thumbwheel position selects thenominal, unloaded speed of the grinder. The thumbwheel is commonlyreferred to as a speed dial, and the position of the speed dial is thespeed setting for the tool.

Certain applications for grinders are best performed at low speed dialsettings. In these applications, as the user may apply more and moreforce, thereby placing the grinder under increasingly heavier loads. Atsome point, which may occur relatively quickly at lower speed dialsettings in comparison with higher speed dial settings, the motor willbegin to slow down as it is simply not capable of delivering that levelof power at the requested speed dial setting. As the motor speeddecreases, the cooling fan attached to the motor shaft also slows down.The airflow generated by the fan drops faster than the speed. As aresult, the cooling of the motor, the gear case, and the electronics,becomes less and less effective. Eventually temperatures may build up tothe point of failure.

One technique for preventing such overheating conditions is through theuse of a temperature sensor disposed inside the housing of the tool.Although not limited hereto, the temperature sensor may be placed in oron a temperature sensitive component of the motor (e.g., a coilwinding). In one embodiment, the controller sets and enforces thecycle-by-cycle current limit in the manner set forth above. Thecontroller may also be configured to receive a signal indicative oftemperature from the temperature sensor. During operation, thecontroller monitors the temperature and further operates to adjust thecycle-by-cycle current limit in accordance with the measured temperatureas shown for example in FIG. 12. In this example, the controllerenforces a 20 Amp current limit so long as the temperature remains below100 Celsius. Once the motor temperature exceeds 100 Celsius, thecontroller lowers the cycle-by-cycle current limit and thereby lowersthe power that can be delivered by the tool. In practice, the user willexperience the reduction in tool performance and back off the forcebeing applied to the tool which in turn allows the motor speed toincrease. As motor speed increase, airflow increases and thereby lowersthe motor temperature. Conversely, as motor temperature decreases, thecontroller increases the cycle-by-cycle current limit until it returnsto the initial 20 Amp limit. Adjustments to the current limit may becorrelated with temperature linearly as shown or in some other mannersuch as exponentially. In this way the power tool user learns themaximum operating point of the grinder at any particular speed dialsetting and can avoid overheating the tool.

Some of the techniques described herein may be implemented by one ormore computer programs executed by one or more processors residing, forexample on a power tool. The computer programs includeprocessor-executable instructions that are stored on a non-transitorytangible computer readable medium. The computer programs may alsoinclude stored data. Non-limiting examples of the non-transitorytangible computer readable medium are nonvolatile memory, magneticstorage, and optical storage.

Some portions of the above description present the techniques describedherein in terms of algorithms and symbolic representations of operationson information. These algorithmic descriptions and representations arethe means used by those skilled in the data processing arts to mosteffectively convey the substance of their work to others skilled in theart. These operations, while described functionally or logically, areunderstood to be implemented by computer programs. Furthermore, it hasalso proven convenient at times to refer to these arrangements ofoperations as modules or by functional names, without loss ofgenerality.

Unless specifically stated otherwise as apparent from the abovediscussion, it is appreciated that throughout the description,discussions utilizing terms such as “processing” or “computing” or“calculating” or “determining” or “displaying” or the like, refer to theaction and processes of a computer system, or similar electroniccomputing device, that manipulates and transforms data represented asphysical (electronic) quantities within the computer system memories orregisters or other such information storage, transmission or displaydevices.

Certain aspects of the described techniques include process steps andinstructions described herein in the form of an algorithm. It should benoted that the described process steps and instructions could beembodied in software, firmware or hardware, and when embodied insoftware, could be downloaded to reside on and be operated fromdifferent platforms used by real time network operating systems.

The foregoing description of the embodiments has been provided forpurposes of illustration and description. It is not intended to beexhaustive or to limit the disclosure. Individual elements or featuresof a particular embodiment are generally not limited to that particularembodiment, but, where applicable, are interchangeable and can be usedin a selected embodiment, even if not specifically shown or described.The same may also be varied in many ways. Such variations are not to beregarded as a departure from the disclosure, and all such modificationsare intended to be included within the scope of the disclosure.

Example embodiments are provided so that this disclosure will bethorough, and will fully convey the scope to those who are skilled inthe art. Numerous specific details are set forth such as examples ofspecific components, devices, and methods, to provide a thoroughunderstanding of embodiments of the present disclosure. It will beapparent to those skilled in the art that specific details need not beemployed, that example embodiments may be embodied in many differentforms and that neither should be construed to limit the scope of thedisclosure. In some example embodiments, well-known processes,well-known device structures, and well-known technologies are notdescribed in detail.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

What is claimed is:
 1. A power tool, comprising: a housing; a brushlesselectric motor drivably connected to an output shaft to impart rotarymotion thereto; a switching arrangement having a plurality of motorswitches and interposed between the electric motor and a power supply;and a controller configured to control a switching operation of theplurality of motor switches for driving the electric motor and enforce acurrent limit on the current delivered to the electric motor, thecontroller configured to operate over periodic time intervals to enforcea current limit on the current delivered to the electric motor, whereinthe controller is configured to enforce the current limit by receiving ameasure of current passing from the power supply to the switchingarrangement within a present time interval of the period time intervals,comparing the measure of instantaneous current to the current limit, andtake corrective action to reduce current delivered to the electric motorif the measure of instantaneous current exceeds the current limit,wherein the controller is further configured to initiate a protectiveresponse if a number of time intervals in which the measure ofinstantaneous current exceeds the current limit exceeds a predeterminedthreshold.
 2. The power tool of claim 1, wherein the protective responsecomprises at least one of: cutting power from the power supply to theelectric motor, or initiating electronic braking of the electric motor.3. The power tool of claim 1, wherein the controller is configured toincrement an event counter by one in response to a determination thatthe measure of instantaneous current exceeds the current limit.
 4. Thepower tool of claim 3, wherein the controller is configured to reset theevent counter to zero in response to a determination that the measure ofinstantaneous current does not exceed the current limit.
 5. The powertool of claim 1, wherein the controller is further configured to monitora rotational speed of the electric motor, and in response to therotational speed of the motor being less than a speed threshold,initiating the protective operation.
 6. The power tool of claim 1,wherein, in response to the measure of instantaneous current exceedingthe current limit within a present time interval of the period timeintervals, the controller turns off the motor switches for a remainderof the present time interval and thereby interrupting current flowing tothe electric motor.
 7. The power tool of claim 6, wherein a duration ofeach time interval of the period time intervals is fixed as a functionof the given frequency at which the plurality of motor switches iscontrolled by the controller.
 8. The power tool of claim 6, wherein thecontroller is configured to turn on select motor switches at the end ofthe present time interval and thereby resume current flow to theelectric motor.
 9. The power tool of claim 6, wherein the duration ofeach time interval is approximately ten times an inverse of the givenfrequency at which the plurality of motor switches is controlled by thecontroller.
 10. The power tool of claim 6, wherein the electric motor iscontrolled by pulse width modulated (PWM) signals and the duration ofthe each time interval corresponds to a period associated with the PWMsignals.
 11. The power tool of claim 1, wherein the brushless electricmotor is further defined as a three-phase DC motor and the switchingarrangement is comprised of six motor switches, each phase of the DCmotor is coupled to a high-side switch and a low-side switch.
 12. Thepower tool of claim 11, wherein the controller enforces the currentlimit using the three low-side switches.
 13. A power tool, comprising: ahousing; a three-phase brushless direct-current (BLDC) electric motordrivably connected to an output shaft to impart rotary motion thereto; aswitching arrangement having a plurality of motor switches andinterposed between the electric motor and a power supply, the pluralityof motor switches comprising three high-side switches and three low-sideswitches, wherein each phase of the electric motor is coupled to one ofthe high-side switches and one of the low-side switches; and acontroller configured to control a switching operation on the threehigh-side switches to control a speed of the electric motor, and enforcea current limit on the current delivered to the electric motor using thethree low-side switches, the controller configured to operate overperiodic time intervals to enforce a current limit on the currentdelivered to the electric motor, wherein the controller is configured toenforce the current limit by receiving a measure of current passing fromthe power supply to the switching arrangement within a present timeinterval of the period time intervals, comparing the measure ofinstantaneous current to the current limit, and take corrective actionto reduce current delivered to the electric motor if the measure ofinstantaneous current exceeds the current limit.
 14. The power tool ofclaim 13, wherein the controller is configured to use a pulse-widthmodulation (PWM) control on the three high-side switches to control thespeed of the electric motor.
 15. The power tool of claim 14, wherein thecontroller is configured to maintain a fixed speed by increasing a dutycycle of PWM control under load.
 16. The power tool of claim 13,wherein, in response to the measure of instantaneous current exceedingthe current limit within a present time interval of the period timeintervals, the controller turns off the motor switches for a remainderof the present time interval and thereby interrupting current flowing tothe electric motor.
 17. The power tool of claim 16, wherein a durationof each time interval of the period time intervals is fixed as afunction of the given frequency at which the plurality of motor switchesis controlled by the controller.
 18. The power tool of claim 16, whereinthe controller is configured to turn on select motor switches at the endof the present time interval and thereby resume current flow to theelectric motor.
 19. The power tool of claim 16, wherein the duration ofeach time interval is approximately ten times an inverse of the givenfrequency at which the plurality of motor switches is controlled by thecontroller.